1. Field of the Invention
The present invention relates to display signal control apparatus provided with methods to avoid burn-in of display apparatus by outputted image signals, display signal control methods, associated programs and storage media.
2. Description of the Related Art
When a display such as a cathode-ray tube (CRT) display, also referred to as the so-called Braun tube, or a plasma display shows the same image continuously for a long period of time, phosphor deterioration occurs only in brightly displayed areas. Consequently, only this part experiences a reduction in the amount of light emitted, generating a so-called burn-in. After prolonged display of the same image on the display apparatus, a viewer will be able to observe this burn-in in the form of unevenness of luminance level. For example, see FIG. 13A.
When the video signal of a broadcast program with an aspect ratio of 4:3 is displayed on a display apparatus with a 16:9 aspect ratio while maintaining a 4:3 aspect ratio, burn-in occurs in the boundary between the image display area and the image non-display area of the display apparatus. In addition, even with a broadcast program with an aspect ratio of 16:9 (known as a full image displaying condition), if an image such as a channel logo or the constant characters, such as a scoreboard of a baseball game, is broadcast for long periods, the patterns of the logo or characters may easily be burnt into the display position. For example, see FIG. 13C.
In addition, there are situations in which the display apparatus shows for long periods images such as an electronic program guide (hereafter referred to as EPG) or a graphic user interface (hereafter referred to as GUI) for various settings and controls. Burn-in occurs frequently in such situations because the EPG and GUI are still images with high contrast.
In order to prevent burn-in during the display of a broadcast program on a display apparatus, there is a method of fixing the display position of a broadcast program and moving it.
Japanese Patent Laid-Open No. 2-268579 discloses a method to shift the display position of an image at regular time intervals. Japanese Patent Laid-Open No. 5-167874 discloses a method to shift the display position of an image when the continuous display time of the image in the specified area exceeds a specified amount of time. Furthermore, Japanese Patent Laid-Open No. 2004-328520 discloses a method to shift the display position of the broadcast program every time the received channel of a television broadcast (broadcast station) is changed, as well as a method to shift the image display position when the difference between the present frame and the previous frame of the video signal is large.
In addition, as a method to prevent burn-in of the EPG and GUI images referred to above, Japanese Patent Laid-Open No. 7-170465 discloses a method of lowering the luminosity of the GUI image of the display apparatus when the GUI image has been displayed for a set amount of time. In addition, as disclosed in Japanese Patent Laid-Open No. 10-63224, there is a method to randomly shift the display position of the GUI image and display luminance level of the display apparatus at each field. Further, as disclosed in Japanese Patent Laid-Open No. 2003-195852, there is a method of shifting the display position of an icon image every time it is displayed.
Although the art described in Japanese Patent Laid-Open Nos. 2-268579 and 5-167874 can prevent burn-in of a display apparatus, they shift the display position of the broadcast program, thereby causing discomfort to viewers when they perceive the shift.
As for the art disclosed in documents such as Japanese Patent Laid-Open No. 2004-328520, in which the broadcast channel display position of the broadcast program is shifted every time the broadcast channel is switched, the display position of the broadcast program is not shifted unless the broadcast channel is changed. Therefore, when viewing a broadcast program on the same channel for a prolonged period, there is the risk of burn-in of the display apparatus. In addition, as disclosed in Japanese Patent Laid-Open No. 2004-328520, when there is a large difference between the previous and present frames, it is possible to prevent burn-in of the display apparatus by switching the displayed screen, i.e., detecting such events as scene changes and shifting the display position of the broadcast program. However, even during one broadcast program there is a plurality of scene changes. For this reason, when there is a scene change, the shifting of the display position of the broadcast program will be perceived by the viewer, e.g., when there has been a movement in the display apparatus of the kind shown in FIGS. 13A to 13B, and FIGS. 13C to 13D.
FIG. 13B shows a situation in which the image in the display apparatus as shown in FIG. 13A has been shifted in an exaggerated fashion from the center to the left side of the display area. In addition, Japanese Patent Laid-Open No. 2004-328520 discloses a technique to prevent burn-in of the display apparatus at the boundary between the video display area and the video non-display area when displaying a side panel or a letter box. In the full image displaying condition, shifting of the display position does not take place. In other words, in this disclosure, there is no way to deal with burn-in of the display apparatus in the full image displaying condition caused by display of channel logos and characters.
On the other hand, the conventional art related to preventing the burn-in of the display apparatus caused by EPG or GUI images, as disclosed in Japanese Patent Laid-Open Nos. 7-170465 and 10-63224, can prevent the burn-in of the display apparatus caused by EPG or GUI images. However, the technique disclosed in Japanese Patent Laid-Open No. 7-170465 causes discomfort in the viewer due to the sudden reduction in luminosity of the EPG or GUI images.
Furthermore, in the technique disclosed by Japanese Patent Laid-Open No. 10-63224, the viewer perceives in units of one field the shifting of the display position or change in luminosity, and this causes discomfort in a similar manner to that caused in the cases of Japanese Patent Laid-Open Nos. 2-268579, 5-167874, and 2004-328520. In addition, in the technique disclosed in Japanese Patent Laid-Open No. 3-195852, if it were assumed that the icon images of this document are EPG or GUI images, it would be necessary to turn the displaying of the images off once and then display them again in order to shift the display position of the images. For this reason, the frequency of shifting the display position would be low, and there would not be a sufficient reduction in the degree of burn-in of the display apparatus. In order to frequently shift the display position of the images, it would be necessary to turn the images on and off repeatedly in a short period of time, and the apparatus would therefore lack severely in user-friendliness.
Accordingly, the present invention has been made in consideration of the above problems, and has as its object to provide a display signal control apparatus, a display signal control method, a program, and a storage medium that are capable of preventing or reducing burn-in of the display apparatus without causing the viewer to feel discomfort while in any viewing mode.